Neuromyelitis optica (NMO) is a severe human demyelinating disorder with predilection for the optic nerve and spinal cord. Approximately 70% of affected individuals demonstrate a serum autoantibody response, NMO-IgG, against the aquaporin-4 (AQP4) water channel. NMO-IgG binding to AQP4 in astrocytes in the central nervous system (CNS) is thought to initiate a series of inflammatory events, including complement- and cell-mediated astrocyte damage, leukocyte recruitment, cytokine release, and myelin damage. Using single- cell FACS and recombinant antibody technology, we have faithfully reconstructed the intrathecal humoral immune response in NMO and demonstrated that AQP4 autoantibodies are sufficient to initiate astrocyte injury and disease pathology. To further the care of patients with human demyelinating disease, we will use our novel resources and innovative approach to address fundamental gaps in our understanding of NMO pathogenesis: 1) Are there alternative targets of the immune response in NMO? 2) Does the nature of the AQP4 immune response correlate with disease severity? 3) What are the mechanisms driving astrocyte injury and myelin damage In NMO pathology? First, to identify novel targets of the autoimmune response in NMO, we will examine non-AQP4 specific NMO recombinant antibodies (rAbs) for binding to human and murine tissue, glial cell lines and primary glial cultures. A compendium of candidate rAbs will be generated from our existing antibody repertoires as well as new cases of AQP4 seronegative disease. Second, to investigate the association between NMO-IgG specificity and clinical disease activity, we will evaluate the relationship between two distinct patterns of AQP4 epitope specificity and the frequency and severity of clinical activity. We hypothesize that the spectrum of AQP4 epitope specificity in NMO-IgG will influence antibody effector function and CNS injury. And third, we will define the mechanisms underlying AQP4-mediated astrocyte injury and myelin damage using a combination of disease models, transgenic animals, and NMO CSF rAbs. Specifically, we will use mixed glial cultures, ex vivo spinal cord explants, and intracerebral injections to evaluate th effect of complement activation, antibody-dependent cell mediated cytotoxicity, excitotoxicity, cytokine release, and astrocyte metabolism on AQP4 antibody-mediated CNS injury. The detailed characterization of the humoral immune response in NMO will aid in the diagnosis and treatment of NMO, the identification of novel target antigens in human demyelinating disease, and the elucidation of the mechanisms causing glial and neuronal injury in CNS inflammatory disease.